Questions every good PHP Developer should be able to answer

Question

I was going through Questions every good  Net developer should be able to answer and was highly impressed with the content and approach of this question and so in the same spirit  I am asking this question for PHP Developer   What questions do you think should a good PHP programmer be able to respond to   EDIT   I am marking this question as community wiki as it is not user specific and it aims to serve programming community at large    Looking forward for some amazing responses    NOTE   Please answer questions too as suggested in the comments so that people could learn something new too regarding the language

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What is the difference between    and     and why would you want to use    at all

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Is php cross-browser    i know  this will make laught many people  but is the more-asked question on php forums

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What is wrong with the following code    a   2  function foo          a   3    foo    echo  a

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Admittedly  I stole this question from somewhere else  can t remember where I read it any more  but thought it was funny   Q  What is T PAAMAYIM NEKUDOTAYIM  A  Its the scope resolution operator  double colon   An experienced PHP er immediately knows what it means  Less experienced  and not Hebrew  developers may want to read this   But more serious questions now     Q  What is the cause of this warning   Warning  Cannot modify header information - headers already sent   and what is a good practice to prevent it  A  Cause  body data was sent  causing headers to be sent too  Prevention  Be sure to execute header specific code first before you output any body data  Be sure you haven t accidentally sent out whitespace or any other characters      Q  What is wrong with this query   SELECT   FROM table WHERE id     POST   id      A  1  It is vulnarable to SQL injection  Never use user input directly in queries  Sanitize it first  Preferebly use prepared statements  PDO  2  Don t select all columns      but specify every single column  This is predominantly ment to prevent queries hogging up memory when for instance a BLOB column is added at some point in the future     Q  What is wrong with this if statement  if   strpos   haystack   needle        A  strpos returns the index position of where it first found the  needle  which could be 0  Since 0 also resolves to false the solution is to use strict comparison  if  false     strpos   haystack   needle         Q  What is the preferred way to write this if statement  and why  if  5     someVar   or if   someVar    5   A  The former  as it prevents accidental assignment of 5 to  someVar when you forget to use 2 equalsigns   someVar   5   and will cause an error  the latter won t     Q  Given this code   function doSomething   amp  arg          return    arg       arg    1      return  return      a   3   b   doSomething   a         what is the value of  a and  b after the function call and why  A   a is 4 and  b is 3  The former because  arg is passed by reference  the latter because the return value of the function is a copy of  not a reference to  the initial value of the argument     OOP specific  Q  What is the difference between public  protected and private in a class definition  A  public makes a class member available to  everyone   protected makes the class member available to only itself and derived classes  private makes the class member only available to the class itself     Q  What is wrong with this code   class SomeClass       protected   someMember       public function   construct                  this- gt  someMember   1             public static function getSomethingStatic                 return  this- gt  someMember   5     here s the catch           A  Static methods don t have access to  this  because static methods can be executed without instantiating a class     Q  What is the difference between an interface and an abstract class  A  An interface defines a contract between an implementing class is and an object that calls the interface  An abstract class pre-defines certain behaviour for classes that will extend it  To a certain degree this can also be considered a contract  since it garantuees certain methods to exist     Q  What is wrong with classes that predominantly define getters and setters  that map straight to it s internal members  without actually having methods that execute behaviour  A  This might be a code smell since the object acts as an ennobled array  without much other use     Q  Why is PHP s implementation of the use of interfaces sub-optimal  A  PHP doesn t allow you to define the expected return type of the method s  which essentially renders interfaces pretty useless   -P

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I d ask something like   a  what about caching   b  how can cache be organised   c  are you sure  you do not do extra DB queries   In my first stuff I ve made on PHP it was a mysql query inside foreach to get names of users who ve made comments    terrible       d  why register globals is evil   e  why and how you should split view from code   f  what is the main aim of  implement    Here are questions that were not clear at all for me after I ve read some basic books  I ve found out all about injections and csx  strpos in a few days weeks through thousands of FAQs in the web  But until I found answers to these questions my code was really terrible

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Why aren t you using something else    Sorry  someone had to say it

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Definitively security questions     simple answers in this post  of course securing php web applications is far more complex    how to deal with SQL injection     mysql real escape string   for a start with MySQL  Then try to learn PDO to take advantage of prepared statements and portability across database vendors    how to deal with CSRF  Cross-Site Request Forgery      Add a token on every important request to secure important operations  user must have seen the form before sending the crucial request      how to deal XSS  Cross-Site Scripting  reflected and stored     htmlentities   is good for a start    variant of XXX injections  LDAP injection  XPath injection  etc        You need to know what is the  vocabulary  used by the XXX and then deduct what you need to sanitize and or  check-and-reject     what is the list of sensible functions     Functions which interpret PHP code  possibly included in a remote file  or which execute command on your system  A short and incomplete list could be  exec    passthru    system    popen    eval    preg replace        how to deal with file inclusion dangers   what is a path transversal   what are the risks associated with file upload     Need careful check of the parameters used when opening file or remote resources    how to enforce the configuration of your PHP configuration  i e  do you know what is the use of php ini      It is going to be long so I skip the answer  please read the PHP manual    about filtering user data  what is the difference between sanitizing and check-and-refuse     The first one transforms the entry in something less hostile  The second one check if the entry is correct and  if not refuse it

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Explain why the following code displays 2 5 instead of 3    a   012  echo  a   4      Answer  When a number is preceded by a 0 in PHP  the number is treated as an octal number  base-8   Therefore the octal number 012 is equal to the decimal number 10

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Explain difference of  extract   explode   implode

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No one touched on it yet but it is something that every PHP developer should be able to speak at length about   Why is register globals bad

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I think a good question would be  how does HTTP work  Working with GET and POST data among other HTTP communications is inherent in PHP development  Understanding how HTTP works in a broader context and how PHP implements this goes a long way

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Terry Chay has a blog post basically summarizing what every PHP developer should know and or be expected to answer to some degree in a job interview   http   terrychay com article php-coders shtml  I think its a great summary

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Why you should never output user input directly   Printing things like data from GET directly can lead to Cross-site scripting  XSS  vulnerabilities  Thats why you should always send input from the client through htmlspecialchars   first

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What is the best practice for escaping user input   This question seems to come up often

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When a site is developed using php and it s utter crap  is it   a  PHPs fault   b  Programmers fault

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What s your favourite debugger    What s your favourite profiler    The actual application ide frontend doesn t matter much as long as it goes beyond  notepad  echo and microtime     It s so unlikely you hire the one in a billion developer that writes perfect code all the time and his her unit tests spotted all the errors and bottlenecks before they even occur that you want someone who can profile and or step through the code and find errors in finite time   That s true for probably all languages platforms but it seems a bit of an underdeveloped skill-set amongst php developers to me  purely subjective speaking

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When calling the  name  element of  array  which is correct      array name   array  name     Both will often work  but only the quoted form is correct   define  name   0   and watch the bugs fly  I ve seen this way too much  How can you force form elements be submitted as an array   Append empty brackets to the name attribute   multiple  lt input type  checkbox  name  checkboxes      gt  elements will be converted to an array on the server  e g    POST  checkboxes   0  n     I don t think it s 100  PHP-specific  but it sure beats looping through   POST for every possible  checkboxes   i element  mysql   mysqli   or PDO   Only one truly wrong answer here  the mysql  library doesn t do prepared statements and can no longer excuse it s capacity for evil   Naming a function  one expected to be called multiple times per executed query   mysql real escape string     is just salt in the wound

[php] Questions every good PHP Developer should be able to answer

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